This used to be possible with Battle.net 1.0 (ie Diablo 2): chat clients could connect to bnet. With the latest round of games it's not supported. I think Blizzard would rather have you immersed in their blockbuster game worlds than using a chat client to talk to your friends. ;)
–
WikwocketMay 29 '12 at 19:44

Maybe that the system evocated here will bring back this feature.
–
AntoMay 30 '12 at 9:47

4 Answers
4

You could ask your friends to add Diablo 3 to Steam or a similar tracking program(Xfire) so you can easily see when they are online, but Blizzard has no official way of seeing if a friend is online outside of the game.

I don't see a way how you could actually check if your friend is online on Diablo 3 when you aren't online - you're supposed to be able to play together with your friend - or at least invite/chat while you are online in Diablo 3.

Besides, what are you doing checking up on your friend if he's playing (or not) Diablo 3 when you aren't playing? Is he grinding faster than you? In that case, that's pretty much illegal and you should be on Diablo 3 24/7 to beat his farming skillz.